The K5 has additional features: the Pro Analysis View and the Custom Mode.
The Pro Analysis View enables you to see line graphs of each breath opposed to bar charts. This is good feedback to enable you to aim for a long flat line on the graph although some people prefer the bars.
The Custom Mode allows you to set a custom number of breaths at a selected load as opposed to the standard 30 breaths. For example you can select maybe 10 breaths on a heavy load or 60 breaths on a light load. This session can be done using the computer or uploaded to the unit. If you go for the K4 you could still do 10 of the 30 breaths and manually select a higher load if you specifically wanted to do this session.